Internal combustion engines generally include one or more valves for controlling the flow of air and fuel through the engine. These valves are usually actuated by a mechanical cam. For example, a rotating shaft having a teardrop shaped cam lobe can be configured to impart motion to the valve, either directly or via one or more intermediate elements. As the shaft rotates, the eccentric portion of the cam lobe imparts a linear motion to the valve over a range of the shaft's rotation.
It can be desirable to alter the valve lift, opening rate, opening timing, closing timing, closing rate, and various other valve parameters to achieve optimum engine efficiency for a variety of operating speeds, loads, temperatures, etc. In addition, in an air hybrid engine in which kinetic energy generated from a vehicle's momentum is recycled using air as the medium, certain hybrid operating modes require that one or more of the engine valves stay open longer or shorter than in other operating modes, and longer or shorter than in a non-hybrid, traditional combustion operating mode.
“Lost-motion” systems have been developed to permit a valve to close earlier than what is called for by the cam. Lost-motion systems generally include a lost-motion valve train element that can be selectively actuated to operatively disconnect a cam from a valve during a portion of the cam's rotation. The motion that would have otherwise been imparted to the valve (had the valve not been operatively disconnected) is thus lost.
Existing lost-motion systems, however, suffer from many shortcomings. For example, the moving components of existing systems are either too heavy or lack the requisite stiffness to be used in high speed and high pressure applications.
Accordingly, there is a need for improved methods and devices for varying the opening and closing parameters of an engine valve.
For purposes of clarity, the term “conventional engine” as used in the present application refers to an internal combustion engine wherein all four strokes of the well-known Otto cycle (the intake, compression, expansion and exhaust strokes) are contained in each piston/cylinder combination of the engine.
Each stroke requires approximately one half revolution of the crankshaft (180 degrees crank angle (CA)), and two full revolutions of the crankshaft (720 degrees CA) are required to complete the entire Otto cycle in each cylinder of a conventional engine.
Also, for purposes of clarity, the following definition is offered for the term “split-cycle engine” as may be applied to engines disclosed in the prior art and as referred to in the present application.
A split-cycle engine generally comprises:
a crankshaft rotatable about a crankshaft axis;
a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft;
an expansion (power) piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft; and
a crossover passage interconnecting the compression and expansion cylinders, the crossover passage including at least a crossover expansion (XovrE) valve disposed therein, but more preferably including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween.
FIG. 1 illustrates a prior art split-cycle, non-hybrid engine. The split-cycle engine 100 replaces two adjacent cylinders of a conventional engine with a combination of one compression cylinder 102 and one expansion cylinder 104. The compression cylinder 102 and the expansion cylinder 104 are formed in an engine block in which a crankshaft 106 is rotatably mounted. The crankshaft 106 includes axially displaced and angularly offset first and second crank throws 126, 128, having a phase angle therebetween. The first crank throw 126 is pivotally joined by a first connecting rod 138 to a compression piston 110, and the second crank throw 128 is pivotally joined by a second connecting rod 140 to an expansion piston 120 to reciprocate the pistons 110, 120 in their respective cylinders 102, 104 in a timed relation determined by the angular offset of the crank throws and the geometric relationships of the cylinders, crank, and pistons. Alternative mechanisms for relating the motion and timing of the pistons can be utilized if desired. The rotational direction of the crankshaft and the relative motions of the pistons near their bottom dead center (BDC) positions are indicated by the arrows associated in the drawings with their corresponding components.
The four strokes of the Otto cycle are thus “split” over the two cylinders 102, 104 such that the compression cylinder 102 contains the intake and compression strokes and the expansion cylinder 104 contains the expansion and exhaust strokes. The Otto cycle is therefore completed in these two cylinders 102, 104 once per crankshaft 106 revolution (360 degrees CA).
During the intake stroke, intake air is drawn into the compression cylinder 102 through an inwardly-opening (opening inward into the cylinder and toward the piston) poppet intake valve 108. During the compression stroke, the compression piston 110 pressurizes the air charge and drives the air charge through a crossover passage 112, which acts as the intake passage for the expansion cylinder 104. The engine 100 can have one or more crossover passages 112.
The volumetric (or geometric) compression ratio of the compression cylinder 102 of the split-cycle engine 100 (and for split-cycle engines in general) is herein referred to as the “compression ratio” of the split-cycle engine. The volumetric (or geometric) compression ratio of the expansion cylinder 104 of the engine 100 (and for split-cycle engines in general) is herein referred to as the “expansion ratio” of the split-cycle engine. The volumetric compression ratio of a cylinder is well known in the art as the ratio of the enclosed (or trapped) volume in the cylinder (including all recesses and open ports) when a piston reciprocating therein is at its bottom dead center (BDC) position to the enclosed volume (i.e., clearance volume) in the cylinder when said piston is at its top dead center (TDC) position. Specifically for split-cycle engines as defined herein, the compression ratio of a compression cylinder is determined when the XovrC valve is closed. Also specifically for split-cycle engines as defined herein, the expansion ratio of an expansion cylinder is determined when the XovrE valve is closed.
Due to very high volumetric compression ratios (e.g., 20 to 1, 30 to 1, 40 to 1, or greater) within the compression cylinder 102, an outwardly-opening (opening outwardly away from the cylinder and piston) poppet crossover compression (XovrC) valve 114 at the crossover passage inlet is used to control flow from the compression cylinder 102 into the crossover passage 112. Due to very high volumetric compression ratios (e.g., 20 to 1, 30 to 1, 40 to 1, or greater) within the expansion cylinder 104, an outwardly-opening poppet crossover expansion (XovrE) valve 116 at the outlet of the crossover passage 112 controls flow from the crossover passage 112 into the expansion cylinder 104. The actuation rates and phasing of the XovrC and XovrE valves 114, 116 are timed to maintain pressure in the crossover passage 112 at a high minimum pressure (typically 20 bar or higher at full load) during all four strokes of the Otto cycle.
At least one fuel injector 118 injects fuel into the pressurized air at the exit end of the crossover passage 112 in coordination with the XovrE valve 116 opening. Alternatively, or in addition, fuel can be injected directly into the expansion cylinder 104. The fuel-air charge fully enters the expansion cylinder 104 shortly after the expansion piston 120 reaches its top dead center (TDC) position. As the piston 120 begins its descent from its TDC position, and while the XovrE valve 116 is still open, one or more spark plugs 122 are fired to initiate combustion (typically between 10 to 20 degrees CA after TDC of the expansion piston 120). Combustion can be initiated while the expansion piston is between 1 and 30 degrees CA past its top dead center (TDC) position. More preferably, combustion can be initiated while the expansion piston is between 5 and 25 degrees CA past its TDC position. Most preferably, combustion can be initiated while the expansion piston is between 10 and 20 degrees CA past its TDC position. Additionally, combustion can be initiated through other ignition devices and/or methods, such as with glow plugs, microwave ignition devices, or through compression ignition methods.
The XovrE valve 116 is closed before the resulting combustion event enters the crossover passage 112. The combustion event drives the expansion piston 120 downward in a power stroke. Exhaust gases are pumped out of the expansion cylinder 104 through an inwardly-opening poppet exhaust valve 124 during the exhaust stroke.
With the split-cycle engine concept, the geometric engine parameters (i.e., bore, stroke, connecting rod length, compression ratio, etc.) of the compression and expansion cylinders are generally independent from one another. For example, the crank throws 126, 128 for the compression cylinder 102 and expansion cylinder 104, respectively, have different radii and are phased apart from one another with TDC of the expansion piston 120 occurring prior to TDC of the compression piston 110. This independence enables the split-cycle engine to potentially achieve higher efficiency levels and greater torques than typical four-stroke engines.
The geometric independence of engine parameters in the split-cycle engine 100 is also one of the main reasons why pressure can be maintained in the crossover passage 112 as discussed earlier. Specifically, the expansion piston 120 reaches its top dead center position prior to the compression piston 110 reaching its top dead center position by a discrete phase angle (typically between 10 and 30 crank angle degrees). This phase angle, together with proper timing of the XovrC valve 114 and the XovrE valve 116, enables the split-cycle engine 100 to maintain pressure in the crossover passage 112 at a high minimum pressure (typically 20 bar absolute or higher during full load operation) during all four strokes of its pressure/volume cycle. That is, the split-cycle engine 100 can be operable to time the XovrC valve 114 and the XovrE valve 116 such that the XovrC and XovrE valves 114, 116 are both open for a substantial period of time (or period of crankshaft rotation) during which the expansion piston 120 descends from its TDC position towards its BDC position and the compression piston 110 simultaneously ascends from its BDC position towards its TDC position. During the period of time (or crankshaft rotation) that the crossover valves 114, 116 are both open, a substantially equal mass of gas is transferred (1) from the compression cylinder 102 into the crossover passage 112 and (2) from the crossover passage 112 to the expansion cylinder 104. Accordingly, during this period, the pressure in the crossover passage is prevented from dropping below a predetermined minimum pressure (typically 20, 30, or 40 bar absolute during full load operation). Moreover, during a substantial portion of the intake and exhaust strokes (typically 90% of the entire intake and exhaust strokes or greater), the XovrC valve 114 and XovrE valve 116 are both closed to maintain the mass of trapped gas in the crossover passage 112 at a substantially constant level. As a result, the pressure in the crossover passage 112 is maintained at a predetermined minimum pressure during all four strokes of the engine's pressure/volume cycle.
For purposes herein, the method of opening the XovrC 114 and XovrE 116 valves while the expansion piston 120 is descending from TDC and the compression piston 110 is ascending toward TDC in order to simultaneously transfer a substantially equal mass of gas into and out of the crossover passage 112 is referred to herein as the “push-pull” method of gas transfer. It is the push-pull method that enables the pressure in the crossover passage 112 of the engine 100 to be maintained at typically 20 bar or higher during all four strokes of the engine's cycle when the engine is operating at full load.
The crossover valves 114, 116 are actuated by a valve train that includes one or more cams (not shown). In general, a cam-driven mechanism includes a camshaft mechanically linked to the crankshaft. One or more cams are mounted to the camshaft, each having a contoured surface that controls the valve lift profile of the valve event (i.e., the event that occurs during a valve actuation). The XovrC valve 114 and the XovrE valve 116 can each have its own respective cam and/or its own respective camshaft. As the XovrC and XovrE cams rotate, eccentric portions thereof impart motion to a rocker arm, which in turn imparts motion to the valve, thereby lifting (opening) the valve off of its valve seat. As the cam continues to rotate, the eccentric portion passes the rocker arm and the valve is allowed to close.
For purposes herein, a valve event (or valve opening event) is defined as the valve lift from its initial opening off of its valve seat to its closing back onto its valve seat versus rotation of the crankshaft during which the valve lift occurs. Also, for purposes herein, the valve event rate (i.e., the valve actuation rate) is the duration in time required for the valve event to occur within a given engine cycle. It is important to note that a valve event is generally only a fraction of the total duration of an engine operating cycle (e.g., 720 degrees CA for a conventional engine cycle and 360 degrees CA for a split-cycle engine).
Further detail on split-cycle engines can be found in U.S. Pat. No. 6,543,225 entitled Split Four Stroke Cycle Internal Combustion Engine and issued on Apr. 8, 2003; U.S. Pat. No. 6,609,371 entitled Split Four Stroke Engine and issued on Aug. 26, 2003; and U.S. Pat. No. 6,952,923 entitled Split-Cycle Four-Stroke Engine and issued on Oct. 11, 2005, each of which is incorporated by reference herein in its entirety.
FIG. 2 illustrates a prior art air hybrid engine in which a split-cycle engine 200 similar to that shown in FIG. 1 is modified to include an air hybrid system. The split-cycle air hybrid engine 200 combines a split-cycle engine with an air reservoir and various controls. This combination enables the engine to store energy in the form of compressed air in the air reservoir. The compressed air in the air reservoir is later used in the expansion cylinder to power the crankshaft.
In general, a split-cycle air hybrid engine as referred to herein comprises:
a crankshaft rotatable about a crankshaft axis;
a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft;
an expansion (power) piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft;
a crossover passage (port) interconnecting the compression and expansion cylinders, the crossover passage including at least a crossover expansion (XovrE) valve disposed therein, but more preferably including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween; and
an air reservoir operatively connected to the crossover passage and selectively operable to store compressed air from the compression cylinder and to deliver compressed air to the expansion cylinder.
Like the engine 100 shown in FIG. 1, the engine 200 includes an engine block 201 having a compression cylinder 202 and an adjacent expansion cylinder 204 extending therethrough. A crankshaft 206 is journaled in the block 201 for rotation about a crankshaft axis. Upper ends of the cylinders 202, 204 are closed by a cylinder head 230.
The first and second cylinders 202, 204 define internal bearing surfaces in which are received for reciprocation a compression piston 210 and a power (or “expansion”) piston 220, respectively. The cylinder head 230, the compression piston 210 and the first cylinder 202 define a variable volume compression chamber 234 in the compression cylinder 202. The cylinder head 230, the power piston 220 and the second cylinder 204 define a variable volume combustion chamber 232 in the power cylinder 204.
The crankshaft 206 includes axially displaced and angularly offset first and second crank throws 226, 228, having a phase angle 236 therebetween. The first crank throw 226 is pivotally joined by a first connecting rod 238 to the compression piston 210, and the second crank throw 228 is pivotally joined by a second connecting rod 240 to the power piston 220 to reciprocate the pistons in their respective cylinders in a timed relation determined by the angular offset of the crank throws and the geometric relationships of the cylinders, crank, and pistons. Alternative mechanisms for relating the motion and timing of the pistons can be utilized if desired. The rotational direction of the crankshaft and the relative motions of the pistons near their bottom dead center (BDC) positions are indicated by the arrows associated in the drawings with their corresponding components.
The cylinder head 230 includes any of various passages, ports, and valves suitable for accomplishing the desired purposes of the split-cycle air hybrid engine 200.
Valves in the cylinder head 230, which are similar to valves of the engine in FIG. 1, include four cam actuated poppet valves: an intake valve 208, an XovrC valve 214, an XovrE valve 216, and an exhaust valve 224. An air reservoir tank valve 252 is also provided. The poppet valves 208, 214, 216, 224 and the air reservoir tank valve 252 can be actuated by camshafts (not shown) having cam lobes for respectively actuating and engaging the valves 208, 214, 216, 224, 252.
A spark plug 222 is mounted in the cylinder head with electrodes extending into the combustion chamber 232 for igniting air fuel charges at precise times by an ignition control, not shown. It should be understood that the engine can also be a diesel engine and be operated without a spark plug. Moreover, the engine 200 can be designed to operate on any fuel suitable for reciprocating piston engines in general, such as hydrogen or natural gas.
The split-cycle air hybrid engine 200 also includes an air reservoir (tank) 242, which is operatively connected to the crossover passage 212 by the air reservoir tank valve 252. Embodiments with two or more crossover passages 212 may include a tank valve 252 for each crossover passage 212, which connect to a common air reservoir 242, or alternatively each crossover passage 212 may operatively connect to separate air reservoirs 242.
The tank valve 252 is typically disposed in an air tank port 254, which extends from the crossover passage 212 to the air tank 242. The air tank port 254 is divided into a first air tank port section 256 and a second air tank port section 258. The first air tank port section 256 connects the air tank valve 252 to the crossover passage 212, and the second air tank port section 258 connects the air tank valve 252 to the air tank 242. The volume of the first air tank port section 256 includes the volume of all additional recesses which connect the tank valve 252 to the crossover passage 212 when the tank valve 252 is closed. Preferably, the volume of the first air tank port section 256 is small relative to the volume of the crossover passage 212 (e.g., less than 25%). More preferably, the first air tank port section 256 is substantially non-existent, that is, the tank valve 252 is most preferably disposed such that it is flush against the outer wall of the crossover passage 212.
The tank valve 252 may be any suitable valve device or system. For example, the tank valve 252 may be a pressure activated check valve, or an active valve which is activated by various valve actuation devices (e.g., pneumatic, hydraulic, cam, electric, or the like). Additionally, the tank valve 252 may comprise a tank valve system with two or more valves actuated with two or more actuation devices.
The air tank 242 is utilized to store energy in the form of compressed air and to later use that compressed air to power the crankshaft 206. This mechanical means for storing potential energy provides numerous potential advantages over the current state of the art. For instance, the split-cycle air hybrid engine 200 can potentially provide many advantages in fuel efficiency gains and NOx emissions reduction at relatively low manufacturing and waste disposal costs in relation to other technologies on the market such as diesel engines and electric-hybrid systems.
The engine 200 typically runs in a normal operating mode (engine firing (EF) mode or sometimes called the normal firing (NF) mode) and one or more air hybrid modes. In the EF mode, the engine 200 functions normally as previously described in detail herein (i.e., with respect to FIG. 1), operating without the use of the air tank 242. In the EF mode, the air tank valve 252 remains closed to isolate the air tank 242 from the basic split-cycle engine. In the four air hybrid modes, the engine 200 operates with the use of the air tank 242.
Exemplary air hybrid modes include:
1) Air Expander (AE) mode, which includes using compressed air energy from the air tank 242 without combustion;
2) Air Compressor (AC) mode, which includes storing compressed air energy into the air tank 242 without combustion;
3) Air Expander and Firing (AEF) mode, which includes using compressed air energy from the air tank 242 with combustion; and
4) Firing and Charging (FC) mode, which includes storing compressed air energy into the air tank 242 with combustion.
Further details on air hybrid engines are disclosed in U.S. Pat. No. 7,353,786 entitled Split-Cycle Air Hybrid Engine and issued on Apr. 8, 2008; U.S. Pat. No. 7,603,970 entitled Split-Cycle Air Hybrid Engine and issued on Oct. 20, 2009; and U.S. Publication No. 2009/0266347 entitled Split-Cycle Air Hybrid Engine and published on Oct. 29, 2009, each of which is incorporated by reference herein in its entirety.
In order to operate the split-cycle engines 100, 200 described above at a high efficiency, a valve actuation system is required that is capable of (1) opening and closing the crossover valves at an extremely high speed, (2) providing a broad range of crossover valve opening and closing timings, and (3) allowing cycle-to-cycle variation in at least the closing timing. These requirements stem from the unique properties of split-cycle engines and, in particular, split-cycle air hybrid engines.
First, in these split-cycle engines, the dynamic actuation of the crossover valves (i.e. 114, 116, 214, 216) is very demanding. This is due to the fact that the crossover valves must achieve sufficient lift to fully transfer the fuel-air charge in a very short period of crankshaft rotation (possibly as little as 6 degrees CA) relative to that of a conventional engine, which normally actuates the valves for a period of at least 180 degrees CA. For example, when operating in EF mode, it is desirable to open the XovrE valve, transfer a fluid charge into the expansion cylinder, and close the XovrE valve while the expansion piston is very close to TDC. Thus, the XovrE valve must typically open and close in a window of about 30 degrees CA to about 35 degrees CA. Under full load conditions, this window is even smaller, perhaps as little as about 10 degrees CA to about 20 degrees CA.
Certain air hybrid modes introduce even more stringent requirements. In AEF mode, for example, a volume of compressed air is stored in the air reservoir 242. Shortly after the expansion piston reaches TDC, the XovrE valve is opened to direct a charge of compressed air (preferably with added fuel) from the reservoir 242 into the combustion chamber where it is then ignited during an expansion stroke. If the engine is operating under only part load and the air reservoir 242 is charged to a high pressure (e.g., above approximately 20 bar), the XovrE valve only needs to be opened for a very short period (e.g., about 6 degrees CA) to transfer the requisite mass of air and fuel into the combustion chamber 232. In other words, the relatively small mass of air-fuel mixture required for part-load operation will quickly flow into the combustion chamber when the air reservoir 242 is charged to a high pressure and therefore the XovrE valve need only open for a few degrees CA. The crossover valves must therefore be capable of actuation rates that are several times faster than the valves of a conventional engine, which means the valve train associated therewith must be stiff enough and at the same time light enough to achieve such fast actuation rates.
Meanwhile, other operating modes may require that the valves stay open for a relatively long period of time. For example, in AE mode, a volume of compressed air stored in the air reservoir 242 is delivered to the combustion chamber 232 without spark or added fuel, forcing the expansion piston down and providing power to the crankshaft. If, however, the air pressure remaining in the reservoir is low (e.g., less than approximately 15 bar) and there is a high torque requirement (e.g., when a vehicle being powered by the engine is accelerating up a hill), the XovrE valve must remain open much longer to allow a sufficient mass of compressed air into the expansion chamber. In some cases, this can be 100 degrees CA or more. Thus, large variations in closing timing are required, since the XovrE valve might need to close 6 degrees CA after opening in one operating mode while it may need to remain open for 100 degrees CA or more in other operating modes, as presented above.
The engines disclosed herein can also require large variations in the opening timing of the crossover valves 214, 216, especially in modes that involve charging the air reservoir (e.g., AC mode and FC mode). In AC mode for instance, the opening timing of the XovrC valve 214 will vary considerably depending on load and the pressure in the air reservoir 242. If the XovrC valve is opened before the pressure in the compression cylinder is greater than or equal to the pressure in the air reservoir, fluid in the air reservoir will undesirably flow back into the compression cylinder 234. The energy required to re-compress this backflow reduces the efficiency of the engine. Therefore, the XovrC valve should not be opened until the pressure in the compression cylinder matches or exceeds that of the air reservoir 242. Thus, a range of approximately 30 to 60 degrees CA of opening timing variability is required for the XovrC valve, depending on the pressure in the air reservoir.
Accordingly, the opening timing, closing timing, and/or various other engine valve parameters must be variable over a wide range of possible values in order to efficiently operate each of the various engine modes.
Moreover, these parameters must be, in some cases, adjustable on a cycle-to-cycle basis. For example, the XovrE valve 216 can be used for load control in operating modes that employ combustion (e.g., EF mode and AEF mode). By closing the XovrE valve at various points along the expansion piston's stroke, the mass of air/fuel supplied to the cylinder can be metered, thereby controlling the engine load. To achieve precise load control in this case, the actuation rate of the XovrE valve must be variable from one cycle to the next.
Existing valve actuation systems are simply incapable of meeting these requirements. They are either too heavy or not stiff enough to be actuated at the required speeds. In addition, they provide only a limited range of opening or closing variability and are not responsive enough for cycle-to-cycle variation.